I honestly don’t know. I spend some time looking at the ACF code and I can’t find a reason that this would be happening.
I can tell you this, since you are using a priority of 10 for your filter and you are using new_post as your post ID then the built in acf filter is running first, it has a priority of 2. The default post status set by acf should be draft.
Additionally, since the acf filter is running first the post ID will never be new_post when your filter is run. The wp_insert_post() portion of your code should never be run.
The only conclusion that I can come up with is that there is some other filter running after yours that is altering the post status again to publish.
